Messages in this thread | | | From | Daniel Phillips <> | Subject | Re: [Linux-cluster] Re: GFS, what's remaining | Date | Sun, 4 Sep 2005 15:51:56 -0400 |
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On Sunday 04 September 2005 03:28, Andrew Morton wrote: > If there is already a richer interface into all this code (such as a > syscall one) and it's feasible to migrate the open() tricksies to that API > in the future if it all comes unstuck then OK. That's why I asked (thus > far unsuccessfully): > > Are you saying that the posix-file lookalike interface provides > access to part of the functionality, but there are other APIs which are > used to access the rest of the functionality? If so, what is that > interface, and why cannot that interface offer access to 100% of the > functionality, thus making the posix-file tricks unnecessary?
There is no such interface at the moment, nor is one needed in the immediate future. Let's look at the arguments for exporting a dlm to userspace:
1) Since we already have a dlm in kernel, why not just export that and save 100K of userspace library? Answer: because we don't want userspace-only dlm features bulking up the kernel. Answer #2: the extra syscalls and interface baggage serve no useful purpose.
2) But we need to take locks in the same lockspaces as the kernel dlm(s)! Answer: only support tools need to do that. A cut-down locking api is entirely appropriate for this.
3) But the kernel dlm is the only one we have! Answer: easily fixed, a simple matter of coding. But please bear in mind that dlm-style synchronization is probably a bad idea for most cluster applications, particularly ones that already do their synchronization via sockets.
In other words, exporting the full dlm api is a red herring. It has nothing to do with getting cluster filesystems up and running. It is really just marketing: it sounds like a great thing for userspace to get a dlm "for free", but it isn't free, it contributes to kernel bloat and it isn't even the most efficient way to do it.
If after considering that, we _still_ want to export a dlm api from kernel, then can we please take the necessary time and get it right? The full api requires not only syscall-style elements, but asynchronous events as well, similar to aio. I do not think anybody has a good answer to this today, nor do we even need it to begin porting applications to cluster filesystems.
Oracle guys: what is the distributed locking API for RAC? Is the RAC team waiting with bated breath to adopt your kernel-based dlm? If not, why not?
Regards,
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